Psych 1XX3 Colour Perception Lecture Notes.pdf

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Psychology 1XX3 Notes – Colour Perception – Mar 7 , 2010
Evolution of Colour Perception:
Who Has Colour Vision?
 Many birds, fish, reptiles, and insects have excellent colour vision. Among
mammals, however, colour vision is limited to primates, including humans.
 This means that your dog and cat, along with the bull, can only see in shades of
grey.
The Functions of Colour Vision in Different Species:
 How did primates end up evolving the ability for colour vision? Primate colour
vision is especially well suited to distinguish red and yellow against a green
background.
 This adaptation helps immensely with foraging for fruit in the bushes and trees.
 In this way, one possible biological advantage of colour vision for primates is the
ability to detect colourful objects in the wild.
 Additionally, the ability to perceive colour is an important advantage for detecting
predators or prey against a background, determining the ripeness of fruit, the
richness of the soil, or even using the colour of sunsets as a means to predict
weather.
The Functions of Colour Vision in Different Species:
 Colour vision is also an important part of the lives of many non-mammalian
species. Many birds, fish and insects are able to see colours that we don’t see at
all, including colours at the UV end of the spectrum.
 For birds, the colour of a potential mate's feathers provides signals to other birds
about how healthy that bird is, and can influence how likely that bird is chosen as
a mate.
 This type of colouration system would help the birds communicate with each
other about how sexy and healthy they are, while still remaining inconspicuous in
the forest to potential predators that are unable to see this bright colouration.
 Next time you see a bee in a flower garden, don’t assume that the beautiful red
rose you see must be irresistible to the bee. 1'he bee might he attracted to flowers
that look dull to human eyes, because it is responding to specific patterns and
colours on the flower that we are unaware of.
 Some flowers have adapted patterns on the petals that are invisible to us, but serve
as "nectar maps” to the bee.
Colour Mixing:
Introduction:
 The human eye processes colour information using principles that have been
known to artists for centuries. You don’t need to have millions of colour receptors
to deal with every conceivable colour out there in the world.
 Instead, you just need a few receptor types whose activity can be combined in
various proportions to make every conceivable colour.
 The three primary colours can be combined in various proportions to make every
colour in the spectrum. Primary colours are like a base colour, they cannot be
reduced into other colours.
 There are actually two different types of colour mixing: additive and subtractive. Def’n of Subtractive Colour Mixing: When coloured pigments selectively absorb some
wavelengths and reflect others
 Subtractive colour mixing applies to the mixing of pigments, dyes, or paints, and
it is called ‘subtractive’ because every reflective surface absorbs (or subtracts) the
colours that it does not reflect. Adding other pigments to that surface alters the
combination wavelengths subtracted.
 So a blue object looks blue to us because all wavelengths are being absorbed by
the object except short, blue waves, which are being reflected back to our eye and
making the object look blue.
 So when we mix two pigments, all wavelengths are being absorbed except those
that the two pigments jointly reflect.
 It may help to think about what would happen if we shine a white light through
both a yellow filter and then a blue filter and look at the remaining light on a
white screen. You would see that it looks green.
 This is because when the light hits the yellow filter all the short waves (the blues
and purples) are being absorbed, or subtracted out, and only the longer (green,
yellow, orange and red) waves are allowed to pass through. When these longer
waves hit the blue filter, it absorbs the longest waves (yellow, orange and red) and
what is left over is a middle band of wavelengths that is transmitted by both
pigments. This middle band that is left over is green.
 With subtractive colour mixing the primary colours are red, yellow and blue,
because these three colours can be mixed in various proportions to make all
colours in the rainbow.
 The complementary colour of red was green, for yellow it was purple, and for
blue it was orange. If you mixed a primary colour with its complementary colour,
you always got brown.
Def’n of Additive Colour Mixing: When coloured lights add their dominant colour to the
mixture.
 The coloured lights add their dominant wavelength to the mixture as opposed to
subtracting wavelengths out.
 This is how our visual system processes colour; by adding the effects of different
wavelengths together in our nervous system. With additive colour mixing, the
primary colours are red, green and blue, because these three colours can be added
together in various proportions to make all the different colours that we see.
 If you made a colour circle using additive colour mixing, you’d find that the
complementary colours are also different: the complementary colour of blue is yellow, for red the complementary colour is a bluish-green, and for green the
complementary colour is a reddish-purple.
 With additive colour mixing, whenever you mix a primary colour with its
complementary colour you get grey or white.
 Take a blue and yellow filter, but instead of passing the light through each filter
successively, overlap the coloured lights from each one onto the reflective
surface.
 Because each coloured light adds its dominant wavelength to the mixture, you
find that now blue and yellow do not make green when added together; but
instead they make grey.
 This is because grey light is the sum of complementary colours (in this case, blue
and yellow).
Theories of Colour Vision:
 Def’n of Trichromatic theory: Proposes that the retina contains three different
kinds of cones.
 The trichromatic theory of colour vision is based on the proposal that the retina
contains three different kinds of receptors that are each maximally sensitive to
different wavelengths of light.
 This very influential theory has a long history, first proposed by Thomas Young
in 1802, and later modified by Hermanvon Helmholtz in 1866. Sometimes it’s
referred to as the Young-Helmholtz theory.
 This theory follows from empirical observations about primary colours and colour
mixing; namely, that it’s possible to match all of the colours of the visible
spectrum by the appropriate mixing of 3 primary colours.  Thus, the trichromatic theory postulated that you only need three different types
of receptors to discriminate all the colours of the visible spectrum. Although this
theory was developed purely from behavioural data, we now know that the human
retina is indeed equipped with three types of cones which contain spectrally
selective photopigments that are maximally responsive to wavelengths of light
that correspond to the primary colours red, green and blue.
 ‘maximally responsive’: this means that a given receptor will respond to other
wavelengths, just not as much as it would to its peak wavelength.
 When you perceive yellow, this is because the red and green cones are equally
stimulated. White is what you see when all three receptors types are stimulated.
 However, some things didn't quite fit the theory. First, yellow seemed to be a
primary colour and not a mixture of red and green. When people were asked to
describe the most basic colours, yellow was usually included as one of them, even
by young children.
 Furthermore, the trichromatic theory could not explain the law of
complementarity, that certain pairs of wavelengths produce the experience of
white.
 Finally, there was the phenomenon of the complementarity of afterimages: why
do you see a yellow afterimage when you stare at a blue stimulus?
The Opponent-Process Theory:
Def’n: Each colour receptor is made up of a pair of opponent colour processes.
 In 1920, Hering formulated the opponent-process theory of colour vision. Like the
trichromatic theory, the opponent- process theory argues that there are three
classes of receptors, but unlike the trichromatic theory, the opponent-process
theory posits that each of these receptors is made up of a pair of opponent
processes.
 Each receptor is capable of being in one of two opponent states and it can only be
in one of those states at a time. The ability to see blues and yellows is mediated by
a blue-yellow opponent receptors, greens and reds are mediated by green-red
opponent receptors, and 3 type of opponent receptor distinguished bright from
dim light; these brightness detectors are excited by lights of any wavelength.
 The new opponent- process was very successful at explaining how a mixture of
wavelengths from complementary colours (like blue and yellow or red and green)
appear white; it also explained why the afterimage of a visual stimulus is the
complementary colour.
 It also fit logically with the fact that most people can easily imagine a yellowish-
red or a bluish-green colour; but it‘s more difficult to imagine a reddish-green or a
bluish- yellow colour.
 According to the opponent-process theory, these colour pairs are opposite and
occur from differential activation of the same receptor type; accordingly, it would
be impossible for a single red-green receptor to be active in both the red and green
states simultaneously.
Both Theories Needed to Explain Colour Perception:
 Both of these theories are needed to explain colour perception. Hurvich and
Jameson elaborated the theories in 1955 and proposed that there are three
component receptors or cones in the retina that are each maximally responsive to
light of a certain wavelength, just as the trichromatic theory suggested.  The three cones are maximally responsive to red, green and blue. The response of
these receptors differentially affect what is happening further down the line in the
brain, where things are organized as the opponent-process theory would have
predicted. The opponent pairs are red-green, blue- yellow, and light-dark.
 The combination of the two theories says that the output of the cones is input for
the next layer of colour processing in the retina, which is organized in an
opponent fashion.
 Colour coding continues in this opponent arrangement up to the brain's visual
processing systems.
 For example, a red light would stimulate a red cone in the retina, which would
then excite the red-green ganglion cells that are organized in an opponent fashion,
and this excitation of the red-green ganglion cell would signal the brain that the
stimulus is red.
 A green light, however, would stimulate a green cone, which